Although Rymer was still at Cambridge in 1662, when he contributed Latin verses to a university volume celebrating the marriage of Charles II and Catherine of Braganza, there is no record of his taking a degree. This may have been due to the financial problems his father was suffering at the time, or the fact that on 13 October 1663 his father was arrested, and executed the following year, for his involvement in the Farnley Wood Plot to stage an uprising in Yorkshire against King Charles II. Although Thomas's elder brother Ralph was also arrested and imprisoned, Thomas himself was not implicated, and on 2 May 1666, he became a member of Gray's Inn, and was called to the bar on 16 June 1673.[3]

His first appearance in print was as translator of René Rapin's Reflections on Aristotle's Treatise of Poesie (1674), to which he added a preface in defence of the classical rules for unity in drama.[4] Following the principles there set forth, he composed a tragedy in verse, licensed 13 September 1677, called Edgar, or the English Monarch, which was a failure. It was printed in 1678.[3]

Rymer's views on the drama were again given to the world in the shape of a printed letter to Fleetwood Shepheard, the friend of Prior, under the title of The Tragedies of the Last Age Consider'd (1678). To Ovid's Epistles Translated by Several Hands (1680), with preface by Dryden, Penelope to Ulysses was contributed by Rymer, who was also one of the hands who Englished the Plutarch of 1683–86. The life of Nicias fell to his share. He furnished a preface to Whitelocke's Memorials of English Affairs (1682), and wrote in 1681 A General Draught and Prospect of the Government of Europe, reprinted in 1689 and 1714 as Of the Antiquity, Power, and Decay of Parliaments, where, ignorant of his future dignity, the critic had the misfortune to observe, "You are not to expect truth from an historiographer royal."

He contributed three pieces to the collection of Poems to the Memory of Edmund Waller (1688), afterwards reprinted in Dryden's Miscellany Poems, and is said to have written the Latin inscription on Waller's monument in Beaconsfield churchyard. The preface to the posthumous Historia Ecclesiastica (1688) of Thomas Hobbes is said to have been by Rymer, but the Life of Hobbes (1681) sometimes ascribed to him was written by Richard Blackburne. He produced a congratulatory poem upon the arrival of Queen Mary in 1689.[3]

His next piece of authorship was to translate the sixth elegy of the third book of Ovid's Tristia for Dryden's Miscellany Poems (1692, p. 148). On the death of Thomas Shadwell in 1692 Rymer received the appointment of historiographer royal, at a yearly salary of £200.[3] Immediately afterwards appeared his much discussed Short View of Tragedy (1693), criticising Shakespeare and Ben Jonson, which produced The Impartial Critick (1693) of John Dennis, the epigram of Dryden.

Rymer's most lasting contribution to scholarship was the sixteen volumes of Fœdera[5] he published from 1704 to 1713; a collection of "all the leagues, treaties, alliances, capitulations, and confederacies, which have at any time been made between the Crown of England and any other kingdoms, princes and states," it was an immense labour of research and transcription on which he spent the last twenty years of his life.[6]